1. Field of the Invention
The present invention relates to a method of designing a spatial phase modulation element and a spatial phase modulation element, and more particularly to a spatial phase modulation element which has a hologram surface and is used for a phase filter, a laser beam shaping device, an optical interconnector, an optical switch, an optical computer, an optical measuring device or the like and a method of designing such a spatial phase modulation element.
2. Description of Prior Art
Conventionally, it is known that optical elements using diffraction are used as optical elements for shaping a laser beam and for diving a laser beam into a plurality of beams. For example, Japanese Patent Laid Open Publication No. 5-323110 proposed an optical element for dividing a laser beam into a plurality of beams.
Also, an optical element which has a variable-index medium (a medium of which refractive index is variable) and a medium with a diffraction surface (hologram surface) is known. The diffraction surface (hologram surface) is located in the interface between the variable-index medium and a nonvariable-index medium (a medium of which refractive index is not variable). This optical element turns on and off its diffraction function by modulating the refractive index of the variable-index medium. More specifically, in such an optical element, generally, when the refractive index of the variable-index medium becomes substantially equal to that of the medium with a diffraction surface (hologram surface), the diffraction function is lost and turned off. Conversely, when the refractive index of the variable-index medium is different from that of the medium with a diffraction surface (hologram surface), the diffraction function is effected and turned on. The modulation of the refractive index is carried out by applying a voltage to the variable-index medium, supplying a current to the medium, heating the medium or the like.
A conventional hologram element using a variable-index medium generates a new pattern designed by a computer when the diffraction function is on and does not generate any new patterns when the diffraction function is off. In this case, with one hologram surface, outputs of two patterns, an output pattern which is the same as an input pattern and an output pattern which is newly generated from an input pattern, are possible.
Conventionally, in order to switch the output of a hologram element between a plurality of output patterns, the same number of hologram surfaces as the number of patterns to be generated are prepared, and the hologram surface to be used is switched between the plurality of hologram surfaces. For example, Japanese Patent Laid Open Publication No. 6-148412 disclosed that two diffraction elements using liquid crystal are laminated and that by switching the elements to be turned on and off, an optical switch, a vari-focus lens or the like can be obtained.
It is more known that by employing a spatial optical modulation element using liquid crystal or the like, any desired hologram surface can be formed, and any desired output pattern can be obtained. In a spatial optical modulation element using liquid crystal, each picture element can be modulated independently, and any desired hologram surface can be formed. However, since each pixel of liquid crystal is of an electrode structure, it is required to have dimensions in a certain extent (in an extent of 30 .mu.m), and when liquid crystal is used for a hologram, a limit must be set to the diffraction angle. Moreover, in order to drive the pixels independently of each other, confusing electrode patterns and a complex control circuit are necessary.
On the other hand, when a plurality of hologram surfaces which were prepared beforehand are switched, the number of possible output patterns depends on the number of the hologram surfaces. Accordingly, in order to generate a large number of output patterns, the number of necessary hologram surfaces is largely increased. In a case wherein a plurality of laminated hologram elements are turned on and off independently of each other to generate a plurality of output patterns, conventionally, the hologram surfaces are designed independently, and only (N+1) output patterns can be generated. Here, N is the number of hologram surfaces. If two or more hologram elements are turned on simultaneously, the output pattern obtained at this time is the one caused by phase modulation by the hologram surfaces of the turned-on hologram elements. However, since each hologram surface is designed independently to generate a desired output pattern, a pattern caused by phase modulation by a plurality of hologram surfaces is out of shape and is different from the desired output pattern. Further, one of the (N+1) output patterns is the same as inputted, and the other N patterns are newly generated patterns.